Lightning has long fascinated the technical community.
Ben Franklin studied lightning's electrical nature over
two centuries ago and Charles R Steinmetz generated
artificial lightning in his General Electric laboratory
in the 1920's. As someone concerned with premises data
communications you need to worry about lightning. Here I
will elaborate on why, where and when you should worry
about lightning. I'll then discuss how to get protection
from it.

6.1 WHY WORRY ABOUT LIGHTNING?

It is unfortunate, but a fact of life, that computers,
computerrelated products and process control equipment
found in premises data communications environments can be
damaged by high-voltage surges and spikes. Such power
surges and spikes are most often caused by lightning
strikes. However, there are occasions when the surges and
spikes result from any one of a variety of other causes.
These causes may include direct contact with
power/lightning circuits, static buildup on cables and
components, high energy transients coupled into equipment
from cables in close proximity, potential differences
between grounds to which different equipments are
connected, miswired systems and even human equipment
users who have accumulated large static electricity
charge build-ups on their clothing. In fact,
electrostatic discharges from a person can produce peak
Voltages up to 15 kV with currents of tens of Amperes in
less than 10 microseconds.

A manufacturing environment is particularly
susceptible to such surges because of the presence of
motors and other high voltage equipment. The essential
point to remember is, the effects of surges due to these
other sources are no different than those due to
lightning. Hence, protection from one will also protect
from the other.

When a lightning-induced power surge is coupled into
your computer equipment any one of a number of harmful
events may occur.

Semiconductors are prevalent in such equipment. A
lightning induced surge will almost always surpass the
voltage rating of these devices causing them to fail.
Specifically, lightning induced surges usually alter the
electrical characteristics of semiconductor devices so
that they no longer function effectively. In a few cases,
a surge may destroy the semiconductor device. These are
called "hard failures." Computer equipment
having a hard failure will no longer function at all. It
must be repaired with the resulting expense of
"downtime" or the expense of a standby unit to
take its place.

In several instances, a lightning-derived surge may
destroy the printed traces in the printed circuit boards
of the computer equipment also resulting in hard
failures.

Along with the voltage source, lightning can cause a
current surge and a resultant induced magnetic field.If
the computer contains a magnetic disk then this
interfering magnetic field might overwrite and destroy
data stored in the disk. Furthermore, the aberrant
magnetic field may energize the disk head when it should
be quiescent. To you, the user, such behavior will be
viewed as the "disk crashing."

Some computer equipment may have magnetic relays. The
same aberrant magnetic fields which cause disk crashes
may activate relays when they shouldn't be activated,
causing unpredictable, unacceptable performance.

Finally, there is the effect of lightning on program
logic controllers (PLCS) which are found in the
manufacturing environment. Many of these PLCs use
programs stored in ROMS. A lightning-induced surge can
alter the contents of the ROM causing aberrant operation
by the PLC.

So these are some of the unhappy things which happen
when a computer experiences lightning. But you may say,
"Come on, equipment hit by lightning, that's like
winning the lottery. It has never happened and I doubt
that it ever will." This is a typical reaction and
unfortunately it is based on ignorance. True, people may
never, or rarely, experience, direct lightning strikes on
exposed, in-building cable feeding into their equipment.
However, it is not uncommon to find computer equipment
being fed by buried cable. In this environment, a
lightning strike, even several miles away, can induce
voltage/current surges which travel through the ground
and induce surges along the cable, ultimately causing
equipment failure. The equipment user is undoubtedly
aware of these failures but usually does not relate them
to the occurrence of lightning during thunderstorm
activity since the user does not experience a direct
strike.

In a way, such induced surges are analogous to chronic
high blood pressure in a person; they are "silent
killers." In the manufacturing environment, long
cable runs are often found connecting sensors, PLCs and
computers. These cables are particularly vulnerable to
induced surges.

6.2 SHOULD YOU WORRY ABOUT LIGHTNING? This
question primarily relates to the geographical location
of computer equipment end-users. When other interfering
phenomena which can cause a deterioration of performance
is considered, it matters little where the equipment is
geographically located.

When do you have to worry during a thunderstorm?
Typically, thunderstorms are characterized as intense
individual rain cells or showers embedded in a broad area
of light rain. These intense cells are only over a fixed
location for a few minutes. They are on average, several
miles in each direction. In the continental United States
thunderstorm cells move from west to east along a squall
line as shown in Figure 17. This squall line is about
12-30 miles in width and up to 1,250 miles long. The
speed at which the thunderstorm cell moves is generally
30 knots (approximately 34.4 statute miles per hour).

6.4 EQUIPMENT PROTECTION

Coming right down to it, a lot can be done as far as
protection is concerned. However, it is best to begin by
describing the magnitude of the threat from which you
need protection.

The first stroke of lightning during a thunderstorm
can produce peak currents ranging from 1,000 to 100,000
Amperes with rise times of 1 microsecond. It is hard to
conceive of, let alone protect against, such enormous
magnitudes. Fortunately, such threats only apply to
direct hits on overhead lines. Hopefully, this is a rare
phenomenon.

More common is the induced surge on a buried cable. In
one test, lightning-induced voltages caused by strokes in
ground flashes at distances of about 5 km were measured
at both ends of a 448 meter long, unenergized power
distribution line.

Typical test results are illustrated in Figure 19.
Notice that the maximum-induced surge exceeds 80 Volts
peak-to-peak. This is more than enough to destroy
semiconductor devices and computer related equipment.
Yet, 80 Volts is well within the range of affordable
protection.

Conceptually, lightning protection devices are
switches to ground. Once a threatening surge is detected,
a lightning protection device grounds the incoming signal
connection point of the equipment being protected. Thus,
redirecting the threatening surge on a path-of-least
resistance (impedance) to ground where it is absorbed.

Any lightning protection device must be composed of
two "subsystems," a switch which is essentially
some type of switching circuitry and a good ground
connection-to allow dissipation of the surge energy. The
switch, of course, dominates the design and the cost.
Yet, the need for a good ground connection can not be
emphasized enough. Computer equipment has been damaged by
lightning, not because of the absence of a protection
device, but because inadequate attention was paid to
grounding the device properly.

Because they can withstand many kilovolts and hundreds
of Amperes, gas tubes have traditionally been used to
suppress lightning surges on telecommunications lines.
This is just what is needed to protect against a direct
strike. Because gas tubes have a relatively slow response
time, this slowness lets enough energy to pass to destroy
typical solid state circuits.

Metal oxide varistors (MOVS)
provide an improvement over the response time problem of
gas tubes. But, operational life is a drawback. MOVs
protection characteristic decays and fails completely
when subjected to prolonged over voltages.

Silicon avalanche diodes have proven to be the most
effective means of protecting computer equipment against
over voltage transients. Silicon avalanche diodes are
able to withstand thousands of high voltage, high current
and transient surges without failure. While they can not
deal with the surge peaks that gas tubes can, silicon
avalanche diodes do provide the fastest response time.
Thus, depending upon the principal threat being protected
against, devices can be found employing gas tubes, MOVS,
or silicon avalanche diodes. This may be awkward, since
the threat is never really known in advance. Ideally, the
protection device selected should be robust, using all
three basic circuit breaker elements. The architecture of
such as device is illustrated in Figure 20. This
indicates triple stage protection and incorporates gas
tubes, MOVs and silicon avalanche diodes as well as
various coupling components and a good ground.

With the architecture shown in Figure 20 a lightning
strike surge will travel, along the line until it reaches
a gas tube. The gas tube dumps extremely high amounts of
surge energy directly to earth ground. However, the surge
rises very rapidly and the gas tube needs several
microseconds to fire.

As a consequence, a delay element is used to slow the
propagation of the leading edge wavefront, thereby
maximizing the effect of the gas tube. For a 90 Volt gas
tube, the rapid rise of the surge will result in its
firing at about 650 Volts. The delayed surge pulse, now
of reduced amplitude, is impressed on the avalanche diode
which responds in about one nanosecond or less and can
dissipate 1,500 Watts while limiting the voltage to 18
Volts for EIA-232 circuits. This 18 Volt level is then
resistively coupled to the MOV which clamps to 27 Volts.
The MOV is additional protection if the avalanche diode
capability is exceeded.

As previously mentioned, the connection to earth
ground can not be over emphasized. The best earth ground
is undoubtedly a cold water pipe. However, other pipes
and building power grounds can also be used. While cold
water pipes are good candidates you should even be
careful here. A plumber may replace sections of corroded
metal pipe with plastic. This would render the pipe
useless as a ground.